In the field of infrared (IR) imaging, the current objective is to provide high pixel count imagers at low cost with high performance. InSb, HgCdTe and quantum well infrared photodetector (QWIP) technologies have demonstrated high performance large area imagers. Each of these technologies has various strengths and weaknesses. InSb photodetectors offer high performance, ease of fabrication, and operation at wavelengths of less than 5 μm, but must be cooled to approximately 80 K. HgCdTe photodetectors can be designed to operate in the important long wavelength IR (LWIR) band corresponding to a wavelength range of 8 to 12 μm and the middle wavelength IR (MWIR) band corresponding to a wavelength range of 3 to 5 μm. HgCdTe photodetectors require very tight tolerances in material and fabrication uniformity, especially in the LWIR band, to ensure high performance. QWIP photodetectors have been demonstrated in both the MWIR and LWIR band. Because of the maturity of the GaAs/AlGaAs material system used in QWIP photodetectors, tight tolerances in both material and fabrication uniformity are readily obtained. QWIP photodetector sensitivity is generally lower than that achieved by InSb or HgCdTe photodetectors in their respective wavelength bands.
Multi-spectral detection is increasingly desirable as a method to increase the probability of detection under various environments. As an example, objects that are only slightly above room temperature, such as a person, are most easily detected in the LWIR corresponding to the peak IR radiation emission band for near room temperature objects. In contrast, a hot object, such as an automobile exhaust pipe, can be readily detected in the MWIR corresponding to the peak IR radiation emission band for objects having a temperature of more than 600 K. Thus, a system that provides high performance with either of these objects should be sensitive to both wavelength bands.
In military applications, it is possible to camouflage an object such that the object emits little radiation in a particular portion of the IR spectrum. A multi-spectral photodetector with appropriately selected sensing wavelengths therefore provides a means of detecting objects that have been camouflaged in this manner.
Additional applications may use multi-spectral photodetectors for discriminating one object from another. As two objects at different temperatures emit different amounts of IR radiation at different wavelengths, a multi-spectral photodetector can more readily discriminate between the objects. As an example, two identical cars may be parked next to each other. If one has recently been driven while the second has not been operated for several hours, a multi-spectral detector could readily sense the subtle differences in emissivities corresponding to temperature differences of less than a degree.
Medical applications can also benefit from the additional discrimination that can be achieved with a multi-spectral photodetector. In particular, detection of cancerous lesions using infrared imaging has shown great promise. The sensitivity of such systems can be increased by imaging at two or more wavelengths to remove any artifacts in the image, such as might be caused by birthmarks, scars, tattoos, etc. The use of two or more wavelengths will also increase sensitivity as smaller temperature differences can be detected.
Sensitivity in these applications can be increased still further by using multi-spectral photodetectors that are sensitive to four or more wavelengths. An imager produced using such multi-spectral photodetectors is effectively an imaging spectrometer. Imaging spectrometers have additional applications including earth imaging and remote sensing. An earth imaging application with an imaging spectrometer can be used to readily detect crop diseases. A military application that would benefit from an imaging spectrometer is mine detection in which earth, disturbed by the placement of a mine, can be readily sensed.
Conventional IR detector technologies have proven difficult to adapt to this current demand for multi-spectral detection. As noted above, high performance single band detectors and imaging arrays have been demonstrated using HgCdTe, InSb and QWIP technologies. Of these, multi-spectral detection has been demonstrated only with the HgCdTe and QWIP technologies. The multi-spectral HgCdTe photodetectors demonstrated to date have excellent performance, but the arrays suffer significantly from both non-uniformity in the HgCdTe material and the fabrication process. While multi-spectral QWIP photodetectors do not place as stringent requirements upon the starting material, the fabrication process has similarly proven to be challenging. Further, both IR detector technologies have suffered from reduced performance in multi-spectral photodetectors in comparison to single band performance. Lastly, operation in more than one wavelength band has generally required at least one additional electrical connection between the photodetector and the external electronics for each wavelength band.
A hyper-spectral IR radiation detector using QWIP photodetector technology is disclosed in U.S. Pat. No. 6,180,990 to Claiborne et al. The disclosed hyper-spectral IR radiation detector uses a linear array of QWIP photodetectors, each absorbing a different wavelength band. To collect information in each of the wavelength bands, the disclosed hyper-spectral IR radiation detector must be scanned across the target. This detection method suffers from possible movement by the target resulting in a loss of spatial registration. Further, as a potentially large number of QWIP photodetectors must be scanned across the image, the resultant information will suffer from a loss of temporal registration as well.
In view of the desirability of multi-spectral IR photodetectors for applications such as an imaging spectrometer, there exists a need for a design that places fewer and/or less stringent requirements upon the starting material and/or the fabrication process. Such photodetectors should also be highly producible. It is also desirable to develop a photodetector technology that requires fewer electrical connections between each photodetector and the external electronics. Furthermore, it is desirable to readily change from detecting in one group of wavelength bands to another group of wavelength bands, even alternating consecutive images between two or more groups of wavelength bands.